The present invention relates to polysaccharides obtained from marine algae, particularly algae belonging to the classes Phaeophyceae, Rhodophyceae and Chlorophyceae, and the antiviral drugs, particularly antiretroviral drugs, more particularly anti-AIDS viral drugs, containing the polysaccharides as active ingredient. The "polysaccharides" referred to in the present invention means a polysaccharide and a protein-bound polysaccharide.
In recent years a new type of irremediable diseases such as AIDS (acquired immunodeficiency syndrome) has been attracted, and the development of therapeutic agents for this disease has been strongly desired. Presently, a nucleic acid derivative called 3'-azido-3'-deoxy thymidine (AZT) is known as the only available therapeutic agent for AIDS. As for the cause of this disease, it has already been disclosed by many researchers that a human immunodeficiency virus (HIV), which is a sort of retrovirus, is adsorbed on a human T.sub.4 lymphocyte to infect this lymphocyte and other lymphocyte one after another until finally the immune system is destroyed.
The present inventors had already found many protein-bound polysaccharides which can serve as a biological resoponse modifier (BRM) for the immune system. For example, they had proposed the following polysaccharides having an anticancer activity in British Patent No. 1,331,513.
Polysaccharides obtained from a liquid extract of a mycelium of a fungus species of Basidiomycetes or from a cultured broth of said species, said polysaccharides being free or substantially free of impurities originally present in said liquid extract or in said cultured broth, and being characterized by being a water-soluble amorphous solid which is non-hydroscopic and non-dialyzable; gives a positive result when tested for the presence of glucose after hydrolysis with 1N sulphuric acid; gives negative results when subjected to the ferric chloride reaction for determining the presence of phenols and to the Fehling reaction for determining the presence of reducing sugars; gives positive results when subjected to the anisaldehyde-sulphuric acid reaction, the Molisch reaction with .alpha.-naphthol, the anthrone-sulphuric acid reaction, the tryptophane-sulphuric acid reaction, the chromotropic acid-sulphuric acid reaction, the aniline-hydrochloric acid reaction, the resorcinol-hydrochloric acid reaction, the carbazole-cysteine-sulphuric acid reaction, the Tollens reaction and the thioglycol-sulphuric acid reaction; shows only one spot at the anode side when subjected to electrophoresis in a 0.05 mol. sodium borate solution for 90 minutes using a cellulose acetate membrane at 20-25 V/cm; and shows no antimicrobial action to bacteria, fungi and yeasts such as Staphylococus aureus, Escherichia coli, Bacillus subtilis, Aspergillus niger and Candida albicans.
The present inventors have noted the fact that the polysaccharide and protein-bound polysaccharide having the function of BRM could exert a great influence to the immune system and, as a result of many studies, it has been found that a substance extracted from marine algae belonging to the classes Phaeophyceae, Rhodophyceae and Chlorophyceae has an inhibiting activity against adsorption of HIV on human-derived lymphocytes and a function to inhibit the activity of reverse transcriptase (RTase) which is an enzyme essential for the proliferation of HIV. The present invention has been attained on the basis of such finding.